P4s in laptops are a bad idea. i know ppl whos laptops crash all the time because of heat problems
I have a P4 3.4 GHz laptop, and it works beautifully. That said, I would still recommend most people consider the Pentium M first.
The reason to get a Pentium 4 has little to do with price and CPU power. It has to do with what components you want inside?
A Pentium 4 box will usually be a larger case that can hold more stuff inside... like 2 hard drives instead of 1, and 2 CD-ROM / DVD drives.
I have a Pentium 4 I just bought in July. I love it. Here's the specs:
Pentium 4 3.4 GHz HT (650 series) 2 MB L2 cache
2048 RAM
dual 80 gig SATA 5400rpm hard drives in RAID 0
256 MB graphics (ATI X800)
17" WSXGA+ wide glassview screen
DVD-R/RW dual layer with LightScribe
It runs Ableton quite well, although a Pentium M laptop usually gets better results in the performance test.
But, you won't find many Pentium M laptops offering dual hard drives, RAID, or dual CD/DVD drives. This may be unimportant to you, though.
So... if you want a big laptop that has dual drives, a Pentium 4 could be right for you. If you just want a well priced laptop that handles Ableton well, then I suggest the Pentium M. You can get a well equiped Pentium M laptop for $1400 or even less. If you want extra options like more RAM, bigger screen, etc... the price will go up... but you shouldn't have to pay more than $2000 for a Pentium M.